1. Field of the Invention
The present invention relates to an operating apparatus of a discharge lamp, and more particularly, relates to an operating apparatus for generating a substantially straight arc discharge by utilizing acoustic resonance in a high intensity discharge (HID) lamp. More specifically, the present invention relates to an operating apparatus of a discharge lamp capable of eliminating a variation in color of the arc discharge caused by cataphoresis and preventing deterioration in the transparency of an arc tube, which is otherwise caused by adhesion of a sealed material in the arc tube to the inner wall of the arc tube in a strip shape surrounding the arc discharge substantially at the center portion of the arc tube, thereby realizing a longer life discharge lamp.
2. Description of the Related Art
An HID lamp has been receiving attention as a light source for exterior illumination or interior illumination, and particularly as a light source for illuminating a commercial store, in view of its high brightness, high efficiency, excellent color rendering property, long life and the like. Recently, a smaller HID lamp which consumes less power has particularly attracted much attention as a light source for a video apparatus or a light source for automobile headlights.
In general, when the above type of discharge lamp is operated while being disposed horizontally, the arc discharge is curved upwardly due to the influence of convection resulting from a temperature distribution generated in the arc tube. When the arc discharge is curved, the high temperature arc discharge of about 5000 K becomes closer to the upper portion of the arc tube. As a result, the temperature of the upper portion of the arc tube becomes higher than that of the lower portion of the arc tube. Therefore, transparency in the upper portion of the arc tube deteriorates faster than that in the lower portion of the arc tube (i.e., transparency is lost). The upper portion of the arc tube also expands thermally. These cause earlier deterioration of the arc tube, adversely influencing the life of the discharge lamp. Particularly in a small low-power HID lamp, the distance between the arc discharge and the arc tube becomes shorter, and thus, the curvature of the arc discharge as described above has greater impact on the life of the discharge lamp.
Moreover, when the arc discharge is curved, the shape of the arc discharge becomes asymmetric between the upper and lower portions of the arc tube. As a result, in the case where the HID lamp is used in combination with a reflecting mirror, such arc curvature must be taken into consideration in optical design. Consequently, such optical design becomes extremely complex and time-consuming.
As a technique for eliminating such arc curvature, methods for operating a discharge lamp by utilizing acoustic resonance are proposed in Japanese Publication for Opposition No. 7-9835 and Japanese Laid-Open Publication No. 7-14684.
Specifically, in Japanese Publication for Opposition No. 7-9835, a discharge lamp is supplied with a current having a waveform as shown in FIG. 13 where an AC current 52 with a frequency which can reduce the influence of convection by means of acoustic resonance to straighten the arc discharge is superposed on a DC current 51. By supplying such a current, the arc curvature is reduced and a substantially straight arc discharge is realized. In FIG. 13, line G represents a ground level, which is also applied to other waveform diagrams used for description of the present invention.
Japanese Laid-Open Publication No. 7-14684 discloses that arc curvature caused by the influence of convection can be eliminated in the following manner. An AC current having such a frequency and waveform in the range of about 10 kHz to about 100 kHz that causes acoustic resonance to be excited in a radial direction in an arc tube is supplied to a discharge lamp. A frequency F.sub.V of the AC current is selected so as to satisfy the following relationship between the frequency F.sub.V of the AC current and a frequency F.sub.R of an acoustic wave in a radial direction: EQU N.multidot.2F.sub.2 =m.multidot.F.sub.R EQU F.sub.R =3.83 C/(2.pi.R)
wherein n and m are integers; C is the velocity of sound in a radial direction in the arc tube; and R is an inner diameter of the arc tube. The publication describes that the arc curvature due to the influence of convection can be eliminated by following the above scheme.
Acoustic resonance is a phenomenon caused by the generation of a standing wave of a compression wave in the arc tube when a natural frequency of a discharge lamp, determined depending on a sealed material in the arc tube and the shape of the arc tube, becomes substantially equal to a frequency of a periodic change in electric power which is input to the discharge lamp. In general, such acoustic resonance causes instability or extinction of arc discharge, bursting of arc tube, and the like. Therefore, conventionally, it has been typically attempted to avoid the generation of acoustic resonance.
In general, acoustic resonance has three kinds of modes, i.e., a radial direction mode, an axial direction mode, and a circumferential direction mode. Among the above three modes, the methods disclosed in Japanese Publication for Opposition No. 7-9835 and Japanese Laid-open Publication No. 7-14684 utilize acoustic resonance in a radial direction.
However, it was found from a study made for the present application that a satisfactory operation state of a discharge lamp is not always realized by the operating method utilizing acoustic resonance disclosed in Japanese Publication for Opposition No. 7-9835 or Japanese Laid-Open Publication No. 7-14684.
More specifically, in the discharge lamp operating apparatus disclosed in Japanese Publication for Opposition No. 7-9835, while the intensity of electric field in the discharge space of the discharge lamp periodically changes in correspondence with the superposed AC current 52, the direction of electric field is maintained in one direction since the current always flows in one direction in the discharge lamp. As a result, cataphoresis occurs where the distribution of a sealed material becomes unbalanced in the arc tube of the discharge lamp, causing variation in the color of the arc discharge generated.
In the discharge lamp operating apparatus disclosed in Japanese Laid-Open Publication No. 7-14684, as schematically shown in FIG. 14, when an arc discharge (not shown) is generated between electrodes 142 and 144, a sealed material in a liquid phase, which does not evaporate, in an arc tube 140 of the discharge lamp adheres to the inner wall of the center portion of the arc tube in a strip shape surrounding the arc discharge (indicated by the reference numeral 146). This phenomenon presumably occurs due to the following reason. While the arc discharge is straightened by acoustic resonance in a radial direction, the density of the sealed material becomes high in the center portion of the arc tube 140 by a different mode of acoustic resonance, i.e., acoustic resonance in an axial direction. The concentrated sealed material thus adheres to the inner wall of the arc tube 140 in a strip shape.
When the sealed material adheres to the inner wall of the center portion of the arc tube 140 in a strip shape surrounding the arc discharge as described above, a chemical reaction between the quartz glass constituting the arc tube 140 and the sealed material is accelerated in the strip-shaped adhesion portion 146. The transparency of the strip-shaped adhesion portion 146 is thus lost, reducing the lumen flux. As a result, the life of the discharge lamp is shortened. In addition, since a discharge lamp is generally used in combination with a reflecting mirror, the light utilizing efficiency of the discharge lamp when combined with the reflecting mirror is reduced if such a strip-shaped adhesion portion 146 where the transparency is lost exists at the center of the arc tube 140.
Japanese Laid-Open Publication No. 10-326681 (corresponding to EP-A-0825808) discloses a technique for suppressing the generation of cataphoresis which arises as problem in Japanese Publication for Opposition No. 7-9835. According to the disclosed technique, a discharge lamp is operated by an operating apparatus which outputs a current having a synthetic wave including a waveform with a frequency component of an acoustic resonance frequency exciting an arc-straightening mode, and a waveform of which the polarity alternately changes at a frequency lower than the acoustic resonance frequency. The publication describes that, using such an operating apparatus, arc curvature is minimized thereby generating a straight arc discharge and variation in the arc color caused by cataphoresis is eliminated.
However, Japanese Laid-Open Publication No. 10-326681 (corresponding to EP-A-0825808) described above fails to consider the problem of adhesion of a sealed material to the inner wall of the center portion of the arc tube. The disclosed technique therefore has the same disadvantages as that described above in relation to Japanese Laid-Open Publication No. 7-14684.